1. Field of the Invention
The present invention relates to an apparatus for measuring the bioelectrical impedance of a living body in which the bioelectrical impedance can be determined by making a weak current of high-frequency flow from one to the other selected point in the living body, and by determining the weak current of high-frequency flowing through the current passage between the opposite selected points and the voltage appearing between another two selected points in the current passage in which the weak current flows.
2. Prior Art
Composition assessment of a selected portion of a living body in terms of its bioelectrical impedance is known from the magazine, xe2x80x9cThe American Journal of Clinical Nutritionxe2x80x9d, 41(4)810-817 1985, xe2x80x9cAssessment of fat-free mass using bioelectrical impedance measurement of the human bodyxe2x80x9d. Also, an apparatus for analyzing the composition in a selected portion of a living body in terms of the bioelectrical impedance appearing in the selected portion is described in the magazine, xe2x80x9cJournal of Applied Physiology VOL77 NO.1, Segmental bioelectrical analysis: theory and application of a new techniquexe2x80x9d. Specifically, the bioelectrical impedance is determined by making electric current of high-frequency flow between the opposite terminal points of both hands and both feet and by measuring the voltage appearing between another two selected points in the current passage in which the weak current flows. A similar apparatus is disclosed in Japanese Patent Application Laid-Open No.10-510455.
In such a conventional bioelectrical impedance gauge a weak current of high-frequency is made to flow in a living body; and the voltage appearing across a given length of current path in which the weak current flows is determined, as seen from FIG. 5.
Referring to FIG. 5, one terminal end of a load Z, which represents a portion selected in a living body, is connected both to the proximal end Ta1 of a high-frequency current supplying cable C1 and to the proximal end Ta4 of a voltage-measuring cable C4 whereas the other terminal end of the load Z is connected both to the proximal end Ta2 of another high-frequency current supplying cable C2 and to the proximal end Ta3 of another voltage-measuring cable C3. Each cable C1, C2, C3 or C4 has its stray capacitance Cs1, Cs2, Cs3 or Cs4 appearing between the cable and the ground. These stray capacitances will have an adverse effect on the measurement.
Different currents appearing on the current supplying side of the impedance gauge of FIG. 5 are given as follows:
I1=I2+Is1
where I1 stands for the electric current flowing from the impedance gauge to the high-frequency current supplying cable C1 (the current being measured by a current detector); I2 stands for the electric current passing through the proximal end Ta1 of the high-frequency current supplying cable C1; and Is1 stands for the electric current flowing through the stray capacitance Cs1 of the high-frequency current supplying cable C1.
The electric current Is4 flows through the stray capacitance Cs4, which appears between the voltage-measuring cable C4 and the ground. The electric current I3 flows in the load. Assuming that the input impedance as seen inward from the voltage measuring terminals N3 and N4 of the impedance gauge is infinitively large, and that the impedance of each cable is zero, the electric current I2 is given by:
I2=I3+Is4. 
Thus, the electric current 13 flowing in the load is given by:
I3=I1xe2x88x92Is1xe2x88x92Is4
The current I1 flowing from the impedance gauge into the high-frequency current supplying cable C1 (measured by the current detector) cannot be equal to the current I3 passing through the impedance Z, thus causing an error in measurement. As a matter of fact, the impedance measured by the impedance gauge is smaller than the impedance of the load Z.
Such an error can be corrected to some extent, but no satisfactory correction can be made; as the cables change in position, their stray capacitances vary. Thus, the measurement will be adversely affected, and the adverse effect is apt to increase with the increase both of the frequency of the electric current and of the cable length.
It takes a significant time for the current signal I1 to flow from one terminal N1 to the other terminal N2 through the out-going high-frequency current supplying cable C1, the load Z and the in-coming high-frequency current supplying cable C2, allowing this delay time to appear as phase lag, which is proportional to the frequency of the electric current flowing in the load Z, thus contributing to the error in the measurement.
In view of the above one object of the present invention is to provide an improved apparatus for measuring the bioelectrical impedance of a living body, which apparatus is guaranteed to be free of the adverse effect caused by the stray capacitances and lengths of associated cables, thus permitting the exact measurement of bioelectrical impedance over an expanded range from low to high frequencies.
To attain this object an apparatus for measuring the bioelectrical impedance of a living body in which the impedance of the living body can be determined by making a weak electric current of high-frequency flow between two selected points on the living body and by determining the voltage between said two selected points or between another two points selected in the current passage in which the weak electric current flows_ is improved according to the present invention in that it comprises: a measuring apparatus including a high-frequency current source, a high-frequency current determining section and a voltage determining section; at least one high-frequency current supplying probe connected to said high-frequency current source via a shielded-conductor cable; a pair of high-frequency current supplying electrodes to be applied to said two selected points; two voltage measuring probes connected to said voltage determining section via shielded-conductor cables; and a pair of voltage measuring electrodes to be applied to said two selected points or said another two points, said high-frequency current supplying probe having a high-frequency current detector connected to and positioned in the vicinity of one of said pair of high-frequency current supplying electrodes.
An apparatus for measuring the bioelectrical impedance of a living body in which the impedance of the living body can be determined by making a weak electric current of high-frequency flow between two selected points on the living body and by determining the voltage between said two selected points or between another two points selected in the current passage in which the weak electric current flows, is improved according to the present invention in that it comprises: a measuring apparatus including a high-frequency current source, a high-frequency current determining section and a voltage determining section; at least one high-frequency current supplying probe connected to said high-frequency current source via a shielded-conductor cable; a pair of high-frequency current supplying electrodes to be applied to said two selected points; two voltage measuring probes connected to said voltage determining section via shielded-conductor cables; and a pair of voltage measuring electrodes to be applied to said two selected points or said another two points, each of said voltage measuring probes having a high input-impedance amplifier connected to and positioned in the vicinity of one or the other voltage measuring electrode.
Two high-frequency current supplying probes may be connected to said high-frequency current source via shielded-conductor cables.
One high-frequency current supplying probe may be connected to said high-frequency current source via a shielded cable, said high-frequency current supplying probe having a high-frequency current detector connected to and positioned in the vicinity of one of the high-frequency current supplying electrodes, and the other high-frequency current supplying electrode being directly connected to said high-frequency current source.
Said high-frequency current detector may comprise a protection circuit connected to one or the other high-frequency current supplying electrode, a reference resistor for detecting the high-frequency current, said reference resistor being connected to said protection circuit at one end, and to said high-frequency current source at the other end via the shielded-conductor cable, and a differential amplifier the input terminals of which are connected across said reference resistor, and the output terminal of which differential amplifier is connected to said high-frequency current determining section via the shielded-conductor cable and an associated impedance matching resistor.
Said high input-impedance amplifier may comprise a protection circuit connected to one or the other voltage measuring electrode, a high-input impedance buffer circuit connected at its input terminal to said protection circuit and at its output terminal to said voltage determining section via the shielded-conductor cable and an associated impedance matching resistor.
All shielded cables may be of same length.
With the arrangements described above the electric current flowing just ahead of one or the other selected point of the living body can be measured, thereby eliminating any errors which otherwise, would be caused by the stray capacitances of the cables. Positioning the high-input impedance buffer circuit close to the voltage determining electrode and making an electric connection thereto effectively minimizes the passage to the infinitely high-impedance input for voltage-representative signals to follow, accordingly reducing the adverse effect caused on the way by the surrounding disturbance or noise signals. The flowing of the impedance-representative currents into the impedance gauge via the impedance-matched, shielded-conductor cables effectively minimizes the adverse effect caused by the surrounding disturbance or noise signals. Finally, use of cables of equal length in which the impedance-representative currents flow makes their transmission time equal, so that the signal delays may be cancelled to eliminate such a phase lag as would be caused if the cables of different lengths were used.